Attendance and absence
You should always notify absences if you are unable to attend classes or other activities related to your education.
The requirements for attendance varies depending on the course. This should be stated in the syllabus and announced at the beginning of each course.
Report absence directly to your teacher or the course coordinator.
Ensemble or other group activity
If you are unable to take part in the work or performance with an ensemble, you have to find a stand-in, so that your absence will not affect the work of the ensemble. Also, be sure to consult with the teacher concerned.